In recent years, a magnetic disk device using a magnetic head for perpendicular magnetic recording has been proposed in order to achieve higher recording density, larger capacity, and miniaturization. In this type of magnetic head, the recording head comprises a main magnetic pole which produces a perpendicular magnetic field, a write shield magnetic pole provided on the trailing side of the main magnetic pole across an intervening write gap, and a coil for supplying a magnetic flux to the main magnetic pole. Further, a recording head of a high-frequency assist recording mode has been proposed, in which a high frequency assist element, for example, a spin torque oscillator, is provided in the write gap between the write shield magnetic pole and the main pole, and a high-frequency field is applied to the magnetic recording layer of a magnetic disk by the spin torque oscillator.
However, in the high frequency assist recording head comprising a spin torque oscillator, while the spin torque oscillator being driven, a return magnetic field from an end portion of the main pole to the write shield magnetic pole side may increase, which, in some cases, cause adverse effect on a recorded signal on an adjacent track aside from the original high-frequency assist effect.